Have Obi-Wan tell Luke right away when they first meet that Vader is his father.

While what I meant was that he’d say it when they’re at his home, I realize that my suggestion could be interpreted as the reveal being literally the first thing Obi-Wan says when meeting Luke in the Jundland Wastes, which would be genuinely hilarious as opposed to just being a spoiler (although it would still be that too).